The advance and development of sophisticated digital communication systems has been accompanied by improvements in signal processing techniques employed at the receiving station, for eliminating or compensating for the effects of transmission distortion that may be introduced into the signal channel. These signal processing techniques include the use of adaptive equalizers or recursive filters through which the characteristics of the channel are estimated and the received data is subjected to a predistortion filter function based upon the observed influence of the channel on the received signals. For a general overview of such signal processing techniques and a description of a particularly advantageous scheme for handling high frequency data and overcoming the signal distortion-introducing characteristics of a dynamic dispersive communication channel, attention is directed to the description in copending application Ser. No. 163,516 filed June 27, 1980 by Daniel D. McRae et al entitled "Technique For High Rate Digital Transmission Over A Dynamic Dispersive Channel," now U.S. Pat. No. 4,365,338, issued Dec. 21, 1982 and assigned to the assignee of the present application. In accordance with the signal recovery technique described in the above-identified application, the disclosure of which is incorporated herein by reference, channel tracking weights of an adaptive transversal filter are updated or refined in accordance with a tracking scheme that simulates the effect of the dispersive medium over which the signals are transmitted. Estimates of the information signals that have been transmitted are produced in accordance with a preselected relationship between prescribed characteristics of the simulated effect of the dynamic dispersive channel and the received signals for successive frames of signals. Through an iterative process the estimates are refined to obtain predictions of actually transmitted data symbols to within a prescribed error tolerance.
Unfortunately, because of the necessary compromise between tracking speeds (loop gain) and stability in feedback systems such as the channel (weighting) tracking loop, the channel weights of such systems do not adjust rapidly enough to cope with fast phase changes due to rapid channel variations, especially during fades, or due to rapid doppler variations, so that, during such conditions there is a degradation in system performance. One way to compensate for rapid changes would be to establish an independent offset tracking function associated with each respective weight. Unfortunately, such an approach introduces noise into the signal prediction process which effectively negates the sought after improvement in system performance. Thus attempting to adjust the changed weights individually does not provide a useful solution to the problem.